Many rubber articles are provided during manufacture with reinforcements. During manufacture the reinforcing material or materials are emplaced in the article prior to curing the rubber to form the finished article. The term "rubber" as herein used includes both natural rubber and synthetics used as rubber substitutes. The term "uncured rubber" refers to rubber which has not been vulcanised but which may include a minor proportion of reclaimed vulcanised or devulcanised rubber which is sometimes incorporated by blending with new uncured rubber. Examples of rubber articles of the type under discussion are tyres and hoses which may be reinforced with steel, woven fabrics, fibres and the like or by a combination of such reinforcements.
In the manufacture of rubber articles, for any of a number of reasons, the article may be scrapped after incorporation of reinforcing materials but prior to vulcanisation. In the case for example of tyre manufacture, there then results a scrap comprising uncured rubber, contaminated with steel, woven fabric and "cured lumps." The scrap is extremely tacky and agglomerates on contact with other like scrap. It is difficult to handle and hitherto has been disposed of by dumping. Not only the uncured rubber but also the metal and fibrous reinforcements would have considerable commercial value if satisfactorily separable one from each other.
Processes are known for the embrittlement of vulcanised articles such as tyres by use of a cryogenic medium to render the rubber frangible followed by impacting to render the frangible rubber into small pieces. In such processes metal is usually recovered from the impacted rubber by magnetic separation. A limited proportion of the fabric contaminated product can be blended with new rubber the majority being fed instead to chemical plants for chemical removal of fibre and for devulcanisation of the rubber.
In one proposed embrittlement process tyres are dipped in a bath of liquid nitrogen. By virtue of the slow rate of progress through baths required to achieve embrittlement, that process requires a large investment of capital in dip-baths and is wasteful of cryogen. More commonly, cooling tunnels divided into a pre-cooling zone and a liquid nitrogen spraying zone have been employed. The tyres are pre-cooled while being conveyed on belts from the feed-point through the pre-cooling zone towards the spraying zone by means of cryogen boiled off in the spraying zone. U.S. Pat. No. 3,992,899 describes a method of embrittling old tyres which avoids some of the disadvantages of the above methods by pre-cooling the tyres in a rotary tunnel inclined downwardly from the feed end and delivering the tyres to a spraying and draining zone. The apparatus provides a high "feed ratio" at the feed end and a long, convoluted pathway for cooling gases from the cryogen at the spraying zone in order to ensure maximum heat transfer and pre-cooling of the feed material. Neither of the foregoing methods provide for segregation of the rubber from fibrous contaminants.
U.K. Pat. No. 1,334,718 describes a process in which fabric and metal contaminants are segregated from vulcanised rubber articles. In that process the vulcanised articles are first embrittled by conventional means. The embrittled vulcanised articles are then disunited from the vulcanised rubber component by flexing, crushing or comminuting the embrittled material. The fabric component is then removed by screening, for preference but not essentially, under cryogenic conditions.
Alternatively, according to the latter patent, the article may be torn to pieces in a cracker mill prior to embrittlement. In that event the embrittled material contains fibre as well as fabric pieces. The embrittled material is then further comminuted prior to segregation and the fibre subsequently removed cyclonically or by screening.
Uncured rubber has chemical and physical properties quite distinct from that of vulcanised rubber. None of the above described processes has been used to reclaim scrap uncured contaminated rubber which has continued to be dumped despite the high commercial incentive for its reclamation.
Uncured rubber is by nature extremely sticky and irretrievably agglomerates on contact with other portions of uncured rubber. Because of those properties uncured rubber cannot be handled in conventional cooling tunnels or through conventional cryogenic baths. Unlike articles of vulcanised rubber such as tyres which have a substantial uniformity of size, scrap arisings of uncured rubber vary drastically in size and for this reason are unsuitable for feeding into apparatus such as has been discussed above. Furthermore because of its extreme stickyness and agglomerative properties, it cannot be pre-comminuted or otherwise reduced to a uniform size suitable for infeed to feeding tunnels or pre-cooling tunnels. In addition unlike embrittled vulcanised rubber, embrittled scrap cannot be allowed to thaw, even locally, at any intermediate process stage since it will then re-agglomerate.
The product from the process of U.K. Pat. No. 1,334,718 is vulcanised rubber which may be blended in limited proportion with new rubber or which is subsequently devulcanised or depolymerised by chemical processes as reclaim. Accordingly a degree of fibre contamination in the product can be tolerated for most purposes. In the recovery of scrap uncured rubber it is desirable that the scrap rubber be obtained substantially free of fibre contamination for direct re-use as new unvulcanised rubber or for blending in any proportion with new unvulcanised rubber and without any further chemical treatment.
That desired degree of freedom from fibre contamination cannot be economically achieved by any of the methods disclosed in U.K. Pat. No. 1,334,718. Even when the problem of embrittling the uncured rubber is solved, the mere screening of impacted contaminated material as taught in that patent is ineffective to efficiently and sufficiently remove fibrous contamination. On the other hand comminution followed by cyclonic separation to remove fibres cannot be conducted efficiently at the low temperatures required to prevent re-agglomeration of the uncured rubber.
The present invention provides a method and apparatus which avoids the above discussed disadvantages of the prior art. While it is especially suitable for use in reclaiming uncured rubber it also has application for reclamation of cured rubber and more generally for separating from each other other materials which are frangible when frozen.